Etch removal of current distribution layer for LED current confinement

ABSTRACT

A method and structure for forming an array of LED devices is disclosed. The LED devices in accordance with embodiments of the invention may include a confined current injection area in which a current spreading layer protrudes away from a cladding layer in a pillar configuration so that the cladding layer is wider than the current spreading layer pillar.

BACKGROUND

Field

The present invention relates to light emitting diode (LED) devices.More particularly, embodiments of the invention relate to LED deviceswith a confined current injection area.

Background Information

Light emitting diodes (LEDs) are increasingly being considered as areplacement technology for existing light sources. For example, LEDs arefound in signage, traffic signals, automotive tail lights, mobileelectronics displays, and televisions. Various benefits of LEDs comparedto traditional lighting sources may include increased efficiency, longerlifespan, variable emission spectra, and the ability to be integratedwith various form factors.

One type of LED is an organic light emitting diode (OLED) in which theemissive layer of the diode is formed of an organic compound. Oneadvantage of OLEDs is the ability to print the organic emissive layer onflexible substrates. OLEDs have been integrated into thin, flexibledisplays and are often used to make the displays for portable electronicdevices such as cell phones and digital cameras.

Another type of LED is a semiconductor-based LED in which the emissivelayer of the diode includes one or more semiconductor-based quantum welllayers sandwiched between thicker semiconductor-based cladding layers.Some advantages of semiconductor-based LEDs compared to OLEDs caninclude increased efficiency and longer lifespan. High luminousefficacy, expressed in lumens per watt (lm/W), is one of the mainadvantages of semiconductor-based LED lighting, allowing lower energy orpower usage compared to other light sources. Luminance (brightness) isthe amount of light emitted per unit area of the light source in a givendirection and is measured in candela per square meter (cd/m²) and isalso commonly referred to as a Nit (nt). Luminance increases withincreasing operating current, yet the luminous efficacy is dependent onthe current density (A/cm²), increasing initially as current densityincreases, reaching a maximum and then decreasing due to a phenomenonknown as “efficiency droop.” Many factors contribute to the luminousefficacy of an LED device, including the ability to internally generatephotons, known as internal quantum efficiency (IQE). Internal quantumefficiency is a function of the quality and structure of the LED device.External quantum efficiency (EQE) is defined as the number of photonsemitted divided by the number of electrons injected. EQE is a functionof IQE and the light extraction efficiency of the LED device. At lowoperating current density (also called injection current density, orforward current density) the IQE and EQE of an LED device initiallyincreases as operating current density is increased, then begins to tailoff as the operating current density is increased in the phenomenonknown as the efficiency droop. At low current density the efficiency islow due to the strong effect of defects or other processes by whichelectrons and holes recombine without the generation of light, callednon-radiative recombination. As those defects become saturated radiativerecombination dominates and efficiency increases. An “efficiency droop”or gradual decrease in efficiency begins as the injection-currentdensity surpasses a low value, typically between 1.0 and 10 A/cm².

Semiconductor-based LEDs are commonly found in a variety ofapplications, including low-power LEDs used as indicators and signage,medium-power LEDs such as for light panels and automotive tail lights,and high-power LEDs such as for solid-state lighting and liquid crystaldisplay (LCD) backlighting. In one application, high-poweredsemiconductor-based LED lighting devices may commonly operate at400-1,500 mA, and may exhibit a luminance of greater than 1,000,000cd/m². High-powered semiconductor-based LED lighting devices typicallyoperate at current densities well to the right of peak efficiency on theefficiency curve characteristic of the LED device. Low-poweredsemiconductor-based LED indicator and signage applications often exhibita luminance of approximately 100 cd/m² at operating currents ofapproximately 20-100 mA. Low-powered semiconductor-based LED lightingdevices typically operate at current densities at or to the right of thepeak efficiency on the efficiency curve characteristic of the LEDdevice. To provide increased light emission, LED die sizes have beenincreased, with a 1 mm² die becoming a fairly common size. Larger LEDdie sizes can result in reduced current density, which in turn may allowfor use of higher currents from hundreds of mA to more than an ampere,thereby lessening the effect of the efficiency droop associated with theLED die at these higher currents.

Thus, the trend in current state-of-the art semiconductor-based LEDs isto increase both the operating current as well as LED size in order toincrease efficiency of LEDs since increasing the LED size results indecreased current density and less efficiency droop. At the moment,commercial semiconductor-based LEDs do not get much smaller than 1 mm².

SUMMARY

Embodiments of the invention describe LED devices with a confinedcurrent injection area. In an embodiment, an LED device includes anactive layer between a first current spreading layer pillar and a secondcurrent spreading layer. The first current spreading layer pillar isdoped with a first dopant type and the second current spreading layer isdoped with a second dopant type opposite the first dopant type. A firstcladding layer is between the first current spreading layer pillar andthe active layer, and a second cladding layer is between the secondcurrent spreading layer and the active layer. The first currentspreading layer pillar protrudes away from the first cladding layer, andthe first cladding layer is wider than the first current spreading layerpillar. In an embodiment, the first current spreading layer pillar isdoped with a p-dopant. In an embodiment, the first current spreadinglayer pillar comprises GaP, and the first cladding layer includes amaterial such as AlInP, AlGaInP, or AlGaAs. In an embodiment, the activelayer includes less than 10 quantum well layers, such as 1-3 quantumwell layers. In an embodiment the active layer includes a single quantumwellayer, and does not include multiple quantum well layers. In anembodiment, the active layer of the LED device has a maximum width of100 μm or less, and the first current spreading layer pillar has amaximum width of 10 μm or less. In an embodiment the active layer of theLED device has a maximum width of 20 μm or less, and the first currentspreading layer pillar has a maximum width of 10 μm or less. In anembodiment, the second current spreading layer is wider than the firstcurrent spreading layer pillar.

A passivation layer may span along a surface of the first cladding layerand sidewalls of the first current spreading layer pillar. In anembodiment, an opening is formed in the passivation layer on a surfaceof the first current spreading layer pillar opposite the first claddinglayer. A conductive contact can then be formed within the opening in thepassivation layer and in electrical contact with the first currentspreading layer pillar without being in direct electrical contact withthe first cladding layer.

In an embodiment, the LED device is supported by a post, and a surfacearea of the top surface of the post is less than the surface area of abottom surface of the first current spreading layer pillar. In such aconfiguration, the LED device may be on a carrier substrate. In anembodiment, the LED device is bonded to a display substrate within adisplay area of the display substrate. For example, the LED device maybe bonded to the display substrate an in electric connection withworking circuitry within the display substrate, or the LED device may bebonded to a display substrate and in electrical connection with a microchip also bonded to the display substrate within the display area. In anembodiment, the LED device is incorporated within a display area of aportable electronic device.

In an embodiment, a method of forming an LED device includes patterninga p-n diode layer of an LED substrate to form an array of currentspreading layer pillars separated by an array of confinement trenches ina current spreading layer of the p-n diode layer, where the confinementtrenches extend through the current spreading layer and expose acladding layer of the p-n diode layer underneath the current spreadinglayer. A sacrificial release layer is formed over the array of currentspreading layer pillars and the cladding layer. The LED substrate isbonded to a carrier substrate, and a handle substrate is removed fromthe LED substrate. The p-n diode layer is patterned laterally betweenthe array of current spreading layer pillars to form an array of LEDdevices, with each LED device including a current spreading layer pillarof the array of current spreading layer pillars. Patterning of the p-ndiode layer may include etching through a top current spreading layer, atop cladding layer, one or more quantum well layers, and the claddinglayer (e.g. bottom cladding layer) to expose the sacrificial releaselayer.

An array of bottom electrically conductive contacts may be formed on andin electrical contact with the array of current spreading layer pillarsprior to forming the sacrificial release layer over the array of currentspreading layer pillars and the cladding layer. The sacrificial releaselayer may additionally be patterned to form an array of openings in thesacrificial release layer over the array of current spreading layerpillars prior to bonding the LED substrate to the carrier substrate. Insuch an embodiment, the LED substrate is bonded to the carrier substratewith a bonding material that is located within the array of openings inthe sacrificial release layer. Upon forming the array of LED devices,the sacrificial release layer may be removed, and a portion of the arrayof LED devices is transferred from the carrier substrate to a receivingsubstrate, for example a display substrate, using an electrostatictransfer head assembly.

In an embodiment, a method of operating a display includes sending acontrol signal to a driving transistor, and driving a current through anLED device including a confined current injection area in response tothe control signal, where the LED device includes a current spreadinglayer pillar that protrudes away from a cladding layer and the claddinglayer is wider than the current spreading layer pillar. For example, thedisplay is a portable electronic device. LED devices in accordance withembodiments of the invention may be driven at injection currents andcurrent densities well below the normal or designed operating conditionsfor standard LEDs. In an embodiment, the current driven through the LEDdevice is from 1 nA-400 nA. In an embodiment the current is from 1 nA-30nA. In such an embodiment, the current density flowing the LED devicemay be from 0.001 A/cm² to 3 A/cm². In an embodiment the current is from200 nA-400 nA. In such an embodiment, the current density flowing theLED device may be from 0.2 A/cm² to 4 A/cm². In an embodiment thecurrent is from 100 nA-300 nA. In such an embodiment, the currentdensity flowing the LED device may be from 0.01 A/cm² to 30 A/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of the relationship of internalquantum efficiency to current density for an LED device in accordancewith embodiments of the invention.

FIG. 2 is a cross-sectional side view illustration of a bulk LEDsubstrate in accordance with an embodiment of the invention.

FIG. 3 is a cross-sectional side view illustration of an array ofcurrent spreading layer confinement trenches formed through a currentspreading layer in accordance with an embodiment of the invention.

FIG. 4 is a cross-sectional side view illustration of a patternedpassivation layer formed over an array of current spreading layerpillars in accordance with an embodiment of the invention.

FIG. 5 is a cross-sectional side view illustration of an array of bottomconductive contacts formed over the array of current spreading layerpillars in accordance with an embodiment of the invention.

FIG. 6 is a cross-sectional side view illustration of a patternedsacrificial release layer formed over the array of current spreadinglayer pillars in accordance with an embodiment of the invention.

FIGS. 7A-7B are cross-sectional side view illustrations of a patternedbulk LED substrate bonded to a carrier substrate with a stabilizationlayer in accordance with embodiments of the invention.

FIG. 8 is a cross-sectional side view illustration of an LED devicelayer and carrier substrate after removal of a handle substrate inaccordance with an embodiment of the invention.

FIG. 9 is a cross-sectional side view illustration of a top conductivecontact layer formed over an LED device layer on a carrier substrate inaccordance with an embodiment of the invention.

FIG. 10 is a cross-sectional side view illustration of an array of mesatrenches formed in the LED device layer to form an array of LED devicesembedded in a sacrificial release layer in accordance with an embodimentof the invention.

FIG. 11A is a cross-sectional side view illustrations of an array of LEDdevices supported by an array of stabilization posts after the removalof a sacrificial release layer in accordance with an embodiment of theinvention.

FIGS. 11B-11D are top-bottom combination schematic view illustrations ofLED devices in accordance with embodiments of the invention.

FIG. 12 is plot of radiative recombination as a function of distancefrom center of LED devices with different widths in accordance with anembodiment of the invention.

FIG. 13 is a plot of internal quantum efficiency as a function ofcurrent density for LED devices with current spreading layer pillars ofdifferent widths in accordance with embodiments of the invention.

FIG. 14 is a plot of internal quantum efficiency as a function ofcurrent density for LED devices with current spreading layer pillars ofdifferent doping in accordance with embodiments of the invention.

FIG. 15A-15E are cross-sectional side view illustrations of an array ofelectrostatic transfer heads transferring LED devices from carriersubstrate to a receiving substrate in accordance with an embodiment ofthe invention.

FIG. 16A is a top view illustration of a display panel in accordancewith an embodiment of the invention.

FIG. 16B is a side-view illustration of the display panel of FIG. 16Ataken along lines X-X and Y-Y in accordance with an embodiment of theinvention.

FIG. 16C is a side-view illustration of an LED device in electricalconnection with a micro chip bonded to a display substrate in accordancewith an embodiment of the invention.

FIG. 17 is a schematic illustration of a display system in accordancewith an embodiment of the invention.

FIG. 18 is a schematic illustration of a lighting system in accordancewith an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe LED devices and manners offorming LED devices with a confined current injection area. Inparticular, some embodiments of the present invention may relate tomicro LED devices and manners of forming micro LED devices with aconfined current injection area.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment”means that a particular feature, structure, configuration, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “in one embodiment” in various places throughout thisspecification are not necessarily referring to the same embodiment ofthe invention. Furthermore, the particular features, structures,configurations, or characteristics may be combined in any suitablemanner in one or more embodiments.

The terms “spanning”, “over”, “to”, “between” and “on” as used hereinmay refer to a relative position of one layer with respect to otherlayers. One layer “spanning,” “over” or “on” another layer or bonded“to” or in “contact” with another layer may be directly in contact withthe other layer or may have one or more intervening layers. One layer“between” layers may be directly in contact with the layers or may haveone or more intervening layers.

In one aspect, embodiments of the invention describe an LED deviceintegration design in which an LED device is transferred from a carriersubstrate and bonded to a receiving substrate using an electrostatictransfer head assembly. In accordance with embodiments of the presentinvention, a pull-in voltage is applied to an electrostatic transferhead in order to generate a grip pressure on an LED device. It has beenobserved that it can be difficult to impossible to generate sufficientgrip pressure to pick up micro devices with vacuum chucking equipmentwhen micro device sizes are reduced below a specific critical dimensionof the vacuum chucking equipment, such as approximately 300 μm or less,or more specifically approximately 100 μm or less. Furthermore,electrostatic transfer heads in accordance with embodiments of theinvention can be used to create grip pressures much larger than the 1atm of pressure associated with vacuum chucking equipment. For example,grip pressures of 2 atm or greater, or even 20 atm or greater may beused in accordance with embodiments of the invention. Accordingly, inone aspect, embodiments of the invention provide the ability to transferand integrate micro LED devices into applications in which integrationis not possible with current vacuum chucking equipment. In someembodiments, the term “micro” LED device or structure as used herein mayrefer to the descriptive size, e.g. length or width, of certain devicesor structures. In some embodiments, “micro” LED devices or structuresmay be on the scale of 1 μm to approximately 300 μm, or 100 μm or lessin many applications. However, it is to be appreciated that embodimentsof the present invention are not necessarily so limited, and thatcertain aspects of the embodiments may be applicable to larger micro LEDdevices or structures, and possibly smaller size scales.

In one aspect, embodiments of the invention describe LED devices thatare poised for pick up and supported by one or more stabilization posts.In accordance with embodiments of the present invention, a pull-involtage is applied to a transfer head in order to generate a grippressure on an LED device and pick up the LED device. In accordance withembodiments of the invention, the minimum amount pick up pressurerequired to pick up an LED device from a stabilization post can bedetermined by the adhesion strength between the adhesive bondingmaterial from which the stabilization posts are formed and the LEDdevice (or any intermediate layer), as well as the contact area betweenthe top surface of the stabilization post and the LED device. Forexample, adhesion strength which must be overcome to pick up an LEDdevice is related to the minimum pick up pressure generated by atransfer head as provided in equation (1):P ₁ A ₁ =P ₂ A ₂  (1)where P₁ is the minimum grip pressure required to be generated by atransfer head, A_(l) is the contact area between a transfer head contactsurface and LED device contact surface, A₂ is the contact area on a topsurface of a stabilization post, and P₂ is the adhesion strength on thetop surface of a stabilization post. In an embodiment, a grip pressureof greater than 1 atmosphere is generated by a transfer head. Forexample, each transfer head may generate a grip pressure of 2atmospheres or greater, or even 20 atmospheres or greater withoutshorting due to dielectric breakdown of the transfer heads. Due to thesmaller area, a higher pressure is realized at the top surface of thecorresponding stabilization post than the grip pressure generate by atransfer head.

In another aspect, embodiments of the invention describe LED devices,which may be micro LED devices, including a confined current injectionarea. In an embodiment, an LED device includes a first (e.g. bottom)current spreading layer pillar doped with a first dopant type, a first(e.g. bottom) cladding layer on the bottom current spreading layer, anactive layer on the bottom cladding layer, a second (e.g top) claddinglayer on the active layer, and a second (e.g. top) current spreadinglayer doped with a second dopant type opposite the first dopant type.The bottom current spreading layer pillar protrudes away from the bottomcladding layer, in which the bottom cladding layer is wider than thebottom current spreading layer pillar. In accordance with embodiments ofthe invention, the active layer is also wider than the bottom currentspreading layer pillar. The top cladding layer, and top currentspreading layer may also be wider than the bottom current spreadinglayer pillar. In this manner, when a potential is applied across the topcurrent spreading layer and bottom current spreading layer pillar, thecurrent injection area within the active layer is modified by therelationship of the areas of the bottom current spreading layer pillarand top current spreading layer. In operation, the current injectionarea is reduced as the area of the bottom current spreading layer pillarconfiguration is reduced. In this manner, the current injection area canbe confined internally within the active layer. Additionally,embodiments of the invention enable the current to be confined withinless than 10 μm from the edge of the current confining feature.Typically, when contact patterning is used to confine current in astandard LED device to prevent injection below optically absorbing metalcontacts, current will still spread greater than 10 μm from the edge ofthe current confining feature. This current spreading distance may bethe entire size of a micro LED device and therefore not a feasibleapproach to current confinement for micro LED devices. Embodiments ofthis invention describe a method to confine current in a micro LEDdevice where confinement distances of less than 10 μm may be needed.

In this manner, it is possible to design an LED device in which a topsurface area of the top surface of the p-n diode layer is larger than asurface area of the bottom current spreading layer pillar. This enableslarger LED devices to be fabricated, which may be beneficial fortransferring the LED devices using an electrostatic transfer headassembly, while also providing a structure in which the confined currentinjection area results in an increased current density and increasedefficiency of the LED device, particularly when operating at injectioncurrents and injection current densities below or near the pre-droopregion of the LED device internal quantum efficiency curve.

In another aspect, it has been observed that non-radiative recombinationmay occur along exterior surfaces of the active layer (e.g. alongsidewalls of the LED devices). It is believed that such non-radiativerecombination may be the result of defects, for example, that may be theresult of forming mesa trenches through the p-n diode layer to form anarray of LED devices or a result of surface states from dangling bondsat the terminated surface that can enable current flow and non-radiativerecombination. Such non-radiative recombination may have a significanteffect on LED device efficiency, particularly at low current densitiesin the pre-droop region of the IQE curve where the LED device is drivenat currents that are unable to saturate the defects. In accordance withembodiments of the invention, the current injection area can be confinedinternally within the active layer, so that the current does not spreadlaterally to the exterior surfaces of the active layer where a largeramount of defects may be present. As a result, the amount ofnon-radiative recombination near the exterior surfaces of the activelayer can be reduced and efficiency of the LED device increased.

The LED devices in accordance with embodiments of the invention arehighly efficient at light emission and may consume very little powercompared to LCD or OLED display technologies. For example, aconventional display panel may achieve a full white screen luminance of100-750 cd/m². It is understood that a luminance of greater than 686cd/m² may be required for sunlight readable screens. In accordance withsome embodiments of the invention, an LED device may be transferred andbonded to a display backplane such as a thin film substrate backplaneused for OLED display panels, where the semiconductor-based LED devicereplaces the organic LED film of the OLED display. In this manner, ahighly efficient semiconductor-based LED device replaces a lessefficient organic LED film. Furthermore, the width/length of thesemiconductor-based LED device may be much less than the allocatedsubpixel area of the display panel, which is typically filled with theorganic LED film.

LED devices in accordance with embodiments of the invention may operatewell below the normal or designed operating conditions for standardLEDs. The LED devices may also be fundamentally different than lasers,and operate at significantly lower currents than lasers. For example,the principle of emission for LED devices in accordance with embodimentsof the invention may be spontaneous, non-directional photon emission,compared to stimulated, coherent light that is characteristic of lasers.Lasers typically include distributed Bragg reflector (DBR) layers onopposite sides of the active layer for stimulating coherent lightemission, also known as lasing. Lasing is not necessary for operation ofLED devices in accordance with embodiments of the invention. As aresult, the LED devices may be thinner than typical lasers, and do notrequire reflector layers on opposite sides of the active layer forstimulating coherent light emission.

For illustrative purposes, in accordance with embodiments of theinvention it is contemplated that the LED devices may be driven using asimilar driving circuitry as a conventional OLED display panel, forexample a thin film transistor (TFT) backplane. However, embodiments arenot so limited. For example, in another embodiment the LED devices aredriven by micro controller chips that are also electrostaticallytransferred to a receiving substrate. Assuming subpixel operatingcharacteristics of 25 nA injection current, an exemplary LED devicehaving a 1 μm² confined current injection area roughly corresponds to acurrent density of 2.5 A/cm², an exemplary LED device having a 25 μm²confined current injection area roughly corresponds to a current densityof 0.1 A/cm², and an exemplary LED device having a 100 μm² confinedcurrent injection area roughly corresponds to a current density of 0.025A/cm². Referring to FIG. 1, in accordance with embodiments of theinvention these low injection currents and current densities maycorrespond to a pre-droop region of a characteristic efficiency curve.This is well below the normal or designed operating conditions forstandard LEDs. Furthermore, in some embodiments, the low injectioncurrents and current densities may correspond to a portion on thepre-droop region of the characteristic efficiency curve for the LEDdevice in which the slope of the curve is greater than 1:1 such that asmall increase in current density results in a greater increase in IQE,and hence EQE, of the LED device. Accordingly, in accordance withembodiments of the invention, significant efficiency increases may beobtained by confining the current injection area of the LED device,resulting in increased luminous efficacy and luminance of the LEDdevice. In some embodiments, LED devices with confined current injectionareas are implemented into display panel applications designed fortarget luminance values of approximately 300 Nit for indoor displayapplications and up to about 2,000 Nit for outdoor display applications.It is to be appreciated that the above examples, including injectioncurrents and display applications are exemplary in nature in order toprovide a context for implementing embodiments of the invention, andthat embodiments are not so limited and may be used with other operatingconditions, and that embodiments are not limited to display applicationsor TFT backplanes.

In the following description exemplary processing sequences aredescribed for forming an array of LED devices, which may be micro LEDdevices. Referring now to FIG. 2, a cross-sectional side viewillustration is provided of a bulk LED substrate 100 in accordance withan embodiment of the invention. For example, the bulk LED substrateillustrated in FIG. 2 may be designed for emission of primary red light(e.g. 620-750 nm wavelength), primary green light (e.g. 495-570 nmwavelength), or primary blue light (e.g. 450-495 nm wavelength), thoughembodiments of the invention are not limited to these exemplary emissionspectra. In an embodiment, a bulk LED substrate 100 includes a p-n diodelayer 115 formed on a growth substrate 102. The p-n diode layer 115 maybe formed of a variety of compound semiconductors having a bandgapcorresponding to a specific region in the spectrum. For example, the p-ndiode layer 115 can include one or more layers based on II-VI materials(e.g. ZnSe) or III-V materials including III-V nitride materials (e.g.GaN, AlN, InN, InGaN, and their alloys) and III-V phosphide materials(e.g. GaP, AlGaInP, and their alloys). The growth substrate 102 mayinclude any suitable substrate such as, but not limited to, silicon,SiC, GaAs, GaN, and sapphire.

Specifically, exemplary primary processing sequences are described forforming an array of red emitting LED devices. While the primaryprocessing sequences are described for red emitting LED devices, it isto be understood that the exemplary processing sequences can be used forLED devices with different emission spectra, and that certainmodifications are contemplated, particularly when processing differentmaterials. Additionally, in different materials the shape of the IQEcurve may differ, specifically the peak may occur at current densitiesother than that shown in FIG. 1. In one embodiment, the bulk LEDsubstrate 100 is designed for emission of red light, and growthsubstrate 102 is formed of GaAs. Growth substrate 102 may optionally bedoped. In the embodiment illustrated growth substrate 102 is n-doped,though in alternative embodiments the growth substrate 102 is p-doped. Acurrent spreading layer 104 is formed on the growth substrate 102 with afirst dopant type. In an embodiment, the current spreading layer 104 isn-doped GaAs, though other materials and opposite dopant types may beused. As illustrated, a cladding layer 106 is formed over the currentspreading layer 104. Cladding layer 106 may function to confine currentwithin the active layer 108, and possess a larger bandgap energy thanthe active layer. The cladding layer 106 may be doped or undoped. In anembodiment, the cladding layer 106 is formed of a material such asAlInP, AlGaInP, or AlGaAs. Cladding layer 106 may optionally be doped orundoped. Cladding layer 106 may optionally be doped, for example withthe same dopant type as current spreading layer 114. For example, dopingof cladding layer 106 may improve vertical current injection into theactive layer 108.

An active layer 108 is formed on the cladding layer 106. The activelayer 108 may include a multi-quantum-well (MQW) configuration or asingle-quantum-well (SQW) configuration. In accordance with embodimentsof the invention, a reduced number of quantum wells may offer moreresistance to lateral current spreading, higher carrier density, and aidin confining current internally within the completed LED device. In anembodiment, the active layer 108 includes a SQW. In an embodiment, a MQWconfiguration with a low number of quantum wells may be used, forexample, for layer quality. In an embodiment, active layer 108 includesa MQW configuration with less than 10 quantum well layers. In anembodiment, active layer 108 includes 1-3 quantum wells. Additionallayers may also be included in the active layer 108, such as one or morebarrier layers. In an embodiment, the active layer 108 is formed of amaterial such as AlGaInP, AlGaAs, or InGaP. In accordance withembodiments of the invention, the materials forming the active layer 108have a smaller bandgap energy than both the cladding layers 106, 110 onopposite sides of the active layer 108.

Still referring to FIG. 2, a cladding layer 110 is formed on the activelayer 108, and a current spreading layer 114 is formed on the claddinglayer 110. In accordance with embodiments of the invention, the claddinglayer 108 material and thickness may be selected to achieve a desiredresistivity at the target operating current so that the cladding layer110 has a higher resistivity than the current spreading layer 114 fromwhich current spreading layer pillars will be formed. In this manner,the cladding layer 110 resists lateral current spreading to a degree sothat current is confined internally within the completed LED device.Similarly as cladding layer 106, cladding layer 110 may function toconfine electrons and holes within the active layer 108, and possess alarger bandgap energy than the active layer. In an embodiment, currentspreading layer 114 is doped with an opposite dopant type than currentspreading layer 104. For example, current spreading layer 114 may bep-doped where current spreading layer 104 is n-doped, and vice versa. Inan embodiment, current spreading layer 114 is GaP. In an embodiment,current spreading layer 114 is formed of multiple layers. In anembodiment, the current spreading layer 114 includes a top p-doped GaPlayer 112 and underlying InGaP etch stop layer 113 on the cladding layer110. In an embodiment, the cladding layer 110 is formed of a materialsuch as AlInP, AlGaInP, or AlGaAs. The cladding layer 110 may be dopedor undoped. Cladding layer 110 may optionally be doped, for example withthe same dopant type as current spreading layer 114. In an embodiment,cladding layer 110 has a lower dopant concentration (including nodoping) than cladding layer 106 dopant concentration.

In an embodiment, bulk LED substrate 100 includes a 250-500 μm thickgrowth substrate 102, a 0.1-1.0 μm thick current spreading layer 104, a0.05-0.5 μm thick cladding layer 106, an active layer 108, a 0.05-5 μmthick cladding layer 110, and a 0.1-1.5 μm thick current spreading layer114. These thicknesses are exemplary, and embodiments of the inventionare not limited to these exemplary thicknesses.

Referring now to FIG. 3 an array of current spreading layer confinementtrenches 116 are formed through a current spreading layer 114 inaccordance with an embodiment of the invention. As shown, the currentspreading layer confinement trenches may be etched completely throughthe current spreading layer 114 forming an array of current spreadinglayer pillars 118. In an embodiment, etching stops on the cladding layer110. In another embodiment, cladding layer 110 is partially etched toensure complete removal of the current spreading layer 114. Inaccordance with embodiments of the invention, etching is stopped beforereaching the active layer 108. Etching may be performed using a suitabletechnique such as wet etching or dry etching techniques. For example,dry etching techniques such as reactive ion etching (RIE),electro-cyclotron resonance (ECR), inductively coupled plasma reactiveion etching (ICP-RIE), and chemically assisted ion-beam etching (CAIBE)may be used. The etching chemistries may be halogen based, containingspecies such as Cl₂, BCl₃, or SiCl₄. The etching chemistries may also bewet chemistries containing species such as Br₂ or HIO₄. In anembodiment, the current spreading layer 114 includes a top p-doped GaPlayer 112 and underlying InGaP etch stop layer 113 on the cladding layer110. In such an embodiment, the top p-doped GaP layer 112 is wet etchedusing a wet etch chemistry containing Br₂ or HIO₄, stopping on an etchstop layer 113 formed of InGaP. The etch stop layer 113 may then beremoved by wet etching in a solution of HCl+H₃PO₄. Alternatively, boththe GaP 112 and InGaP 113 layers can be etched using a timed dry etchingtechnique.

As will become more apparent in the following description, the width ofthe current spreading layer pillars 118 at least partly determines theability to increase current density within the LED device as well as theability to confine current internally within the LED devices and awayfrom the external sidewalls where non-radiative recombination may occur.While some lateral current spreading occurs within the device,embodiments of the invention generally refer to the confined currentarea as the area of the quantum well directly above the currentspreading layer pillars 118. Width of the current spreading layerpillars 118 may also be related to width of the LED devices. In someembodiments, current spreading layer pillars 118 have a width between 1and 10 μm. In an embodiment, current spreading layer pillars 118 have awidth or diameter of approximately 2.5 μm.

FIG. 4 is a cross-sectional side view illustration of a patternedpassivation layer 120 formed over an array of current spreading layerpillars 118 in accordance with an embodiment of the invention. In anembodiment, a passivation layer 120 is formed of an electricallyinsulating material such as an oxide or nitride. In an embodiment,passivation layer is approximately 50 angstroms to 3,000 angstroms thickAl₂O₃. In an embodiment, passivation layer 120 is formed using a highquality thin film deposition procedure, such as atomic layer deposition(ALD). As will become more apparent in the following description, a highquality thin film deposition procedure may protect the integrity of thepassivation layer 120 during the sacrificial release layer etchoperation. In an embodiment, passivation layer 120 is approximately 200angstroms thick Al₂O₃ deposited by ALD. Openings 122 may then be formedover the current spreading layer pillars 118 to expose the top-mostsurface of the current spreading layer pillars using a suitablepatterning technique such as lithography and etching. In the embodimentillustrated, patterned passivation layer 120 is formed along sidewallsof current spreading layer pillars 118 and on cladding layer 110. Inother embodiments, a passivation layer 120 is not formed.

Referring now to FIG. 5, an array of bottom conductive contacts 124 areformed over the array of current spreading layer pillars 118 inaccordance with an embodiment of the invention. Conductive contacts 124may be formed of a variety of conductive materials including metals,conductive oxides, and conductive polymers. In an embodiment, conductivecontacts 124 are formed using a suitable technique such as evaporationor sputtering. In an embodiment, conductive contacts 124 may includeBeAu metal alloy, or a metal stack of Au/GeAu/Ni/Au layers. In anembodiment, conductive contacts 124 include a first layer to make ohmiccontact with current spreading layer pillars 118, and a secondbonding-release layer such as gold to control adhesion with astabilization layer used to bond to a carrier substrate. Following theformation of the bottom conductive contacts 124, or at least the ohmiclayer, the substrate stack may be annealed to make ohmic contact, forexample, at 510° C. for 10 minutes. In the embodiment illustrated inFIG. 5, conductive contacts 124 do not completely span between adjacentcurrent spreading layer pillars 118. In an embodiment, conductivecontacts 124 span along the sidewalls of the current spreading layerpillars 118 covered by passivation layer 120. In an embodiment,conductive contacts 124 do not span along the sidewalls of the currentspreading layer pillars 118.

A sacrificial release layer 126 may then be formed over the array ofcurrent spreading layer pillars 118 as illustrated in FIG. 6. In theparticular embodiment illustrated, the sacrificial release layer 126 isformed within current confinement trenches 116. In an embodiment, thesacrificial release layer 126 is formed of a material which can bereadily and selectively removed with vapor (e.g. vapor HF) or plasmaetching. In an embodiment, the sacrificial release layer is formed of anoxide (e.g. SiO₂) or nitride (e.g. SiN_(x)), with a thickness of 0.2 μmto 2 μm. In an embodiment, the sacrificial release layer is formed usinga comparatively low quality film formation technique compared to thepassivation layer 120. In an embodiment, the sacrificial release layer126 is formed by sputtering, low temperature plasma enhanced chemicalvapor deposition (PECVD), or electron beam evaporation.

Still referring to FIG. 6, the sacrificial release layer 126 ispatterned to from an array of openings 128 over the array of currentspreading layer pillars 118. In an embodiment, each opening 128 exposesan underlying conductive contact 124. As will become more apparent inthe following description, the dimensions of the openings 128 in thesacrificial release layer 126 correspond to the dimensions and contactarea of the stabilization posts to be formed, and resultantly to theadhesion strength that must be overcome to pick up the array of LEDdevices that is supported by and poised for pick from the array ofstabilization posts. In an embodiment, openings 128 are formed usinglithographic techniques and have a length and width of approximately 0.5μm by 0.5 μm, though the openings may be larger or smaller. In anembodiment, openings 128 have a width (or area) that is less than thewidth (or area) of the current spreading layer pillars 118.

Referring now to FIGS. 7A-7B, in some embodiments a stabilization layer130 is formed over the patterned sacrificial release layer 126 and thepatterned bulk LED substrate 100 is bonded to a carrier substrate 140.In accordance with embodiments of the invention, stabilization layer 130may be formed of an adhesive bonding material. In an embodiment theadhesive bonding material is a thermosetting material such asbenzocyclobutene (BCB) or epoxy. For example, the thermosetting materialmay be associated with 10% or less volume shrinkage during curing, ormore particularly about 6% or less volume shrinkage during curing so asto not delaminate from the conductive contacts 124 on the LED devices tobe formed. In order to increase adhesion the underlying structure can betreated with an adhesion promoter such as AP3000, available from The DowChemical Company, in the case of a BCB stabilization layer in order tocondition the underlying structure. AP3000, for example, can be spincoated onto the underlying structure, and soft-baked (e.g. 100° C.) orspun dry to remove the solvents prior to applying the stabilizationlayer 130 over the patterned sacrificial release layer 126.

In an embodiment, stabilization layer 130 is spin coated or spray coatedover the patterned sacrificial release layer 126, though otherapplication techniques may be used. Following application of thestabilization layer 130, the stabilization layer may be pre-baked toremove the solvents. After pre-baking the stabilization layer 130 thepatterned bulk substrate 100 is bonded to the carrier substrate 140 withthe stabilization layer 130. In an embodiment, bonding includes curingthe stabilization layer 130. Where the stabilization layer 130 is formedof BCB, curing temperatures should not exceed approximately 350° C.,which represents the temperature at which BCB begins to degrade.Achieving a 100% full cure of the stabilization layer may not berequired in accordance with embodiments of the invention. In anembodiment, stabilization layer 130 is cured to a sufficient curingpercentage (e.g. 70% or greater for BCB) at which point thestabilization layer 130 will no longer reflow. Moreover, it has beenobserved that partially cured BCB may possess sufficient adhesionstrengths with carrier substrate 140 and the patterned sacrificialrelease layer 126. In an embodiment, stabilization layer may besufficiently cured to sufficiently resist the sacrificial release layerrelease operation.

In an embodiment, the stabilization layer 130 is thicker than the heightof the current spreading layer pillars 118 and openings 128 in thepatterned sacrificial release layer 126. In this manner, the thicknessof the stabilization layer filling openings 128 will becomestabilization posts 132, and the remainder of the thickness of thestabilization layer 130 over the filled openings 128 can function toadhesively bond the patterned bulk LED substrate 100 to a carriersubstrate 140.

In the embodiment illustrated in FIG. 7A, after bonding to the carriersubstrate 140 a continuous portion of stabilization layer 130 remainsover the carrier substrate 140. In an embodiment illustrated in FIG. 7B,the sacrificial release layer 126 (or another intermediate layer) ispressed against the carrier substrate 140 during bonding such that thereis not a thickness of the stabilization layer 130 below thestabilization posts 132 to be formed. In such an embodiment, theconfinement trenches 116 can function as overflow cavities for thestabilization layer during bonding.

Following bonding of the patterned bulk LED substrate 100 to the carriersubstrate 140, the handle substrate 102 is removed as illustrated inFIG. 8. Removal of handle substrate 102 may be accomplished by a varietyof methods including laser lift off (LLO), grinding, and etchingdepending upon the material selection of the growth substrate 102. Inthe particular embodiment illustrated where handle substrate 102 is agrowth substrate formed of GaAs, removal may be accomplished by etching,or a combination of grinding and etching. For example, the GaAs growthsubstrate 102 can be removed with a H₂SO₄+H₂O₂ solution, NH₄OH+H₂O₂solution, or CH₃OH+Br₂ chemistry.

Referring now to FIG. 9, following the removal of the growth substrate102 a top conductive contact layer 152 may be formed. Top conductivecontact layer 152 may be formed of a variety of electrically conductivematerials including metals, conductive oxides, and conductive polymers.In an embodiment, conductive contact layer 152 is formed using asuitable technique such as evaporation or sputtering. In an embodiment,conductive contact layer 152 is formed of a transparent electrodematerial. Conductive contact layer 152 may include BeAu metal alloy, ora metal stack of Au/GeAu/Ni/Au layers. Conductive contact layer 152 mayalso be a transparent conductive oxide (TCO) such as indium-tin-oxide(ITO). Conductive contact layer 152 can also be a combination of one ormore metal layers and a conductive oxide. In an embodiment, conductivecontact layer 152 is approximately 300 angstroms thick ITO. In anembodiment, after forming the conductive contact layer 152, thesubstrate stack is annealed to generate an ohmic contact between theconductive contact layer and the current spreading layer 104. Where thestabilization layer 130 is formed of BCB, the annealing temperature maybe below approximately 350° C., at which point BCB degrades. In anembodiment, annealing is performed between 200° C. and 350° C., or moreparticularly at approximately 320° C. for approximately 10 minutes.

In an embodiment, prior to forming the top conductive contact layer 152an ohmic contact layer 150 can optionally be formed to make ohmiccontact with the current spreading layer 104. In an embodiment, ohmiccontact layer 150 may be a metallic layer. In an embodiment, ohmiccontact layer 150 is a thin GeAu layer. For example, the ohmic contactlayer 150 may be 50 angstroms thick. In the particular embodimentillustrated, the ohmic contact layer 150 is not formed directly over thecurrent spreading layer pillars 118, corresponding to the currentconfinement area within the LED devices, so as to not reflect light backinto the LED device and potentially reduce light emission. In someembodiments, ohmic contact layer 150 forms a ring around the currentspreading layer pillars 118.

Referring now to FIG. 10, an array of mesa trenches 154 is formed in theLED device layer 115 to form an array of LED devices 156 embedded in thesacrificial release layer in accordance with an embodiment of theinvention. In the embodiment illustrated, mesa trenches 154 extendthrough the top conductive contact layer 152 and LED device layer 115laterally between the array of current spreading layer pillars 118stopping on the sacrificial release layer to form an array of LEDdevices 156. As illustrated, each LED device 156 includes mesa structurewith sidewalls 168 formed through the device layer 115 and a currentspreading layer pillar 118 of the array of current spreading layerpillars. In an embodiment, current spreading layer pillars 118 arecentrally located in the middle of the LED devices 156 so as to confinecurrent equally from the sidewalls 168 of the LED devices 156. At thispoint, the resultant structure is still robust for handling and cleaningoperations to prepare the substrate for subsequent sacrificial layerremoval and electrostatic pick up. Etching may be performed using asuitable technique such as dry etching. For example, dry etchingtechniques such as reactive ion etching (RIE), electro-cyclotronresonance (ECR), inductively coupled plasma reactive ion etching(ICP-RIE), and chemically assisted ion-beam etching (CAIBE) may be used.The etching chemistries may be halogen based, containing species such asCl₂, BCl₃, or SiCl₄. In an embodiment, etching is continued throughpassivation layer 120, stopping on the sacrificial release layer 126.

Still referring to FIG. 10, in an embodiment the top conductive contacts152 on each LED device 156 cover substantially the entire top surface ofeach LED device 156. In such a configuration, the top conductivecontacts 152 cover substantially the maximum available surface area toprovide a large, planar surface for contact with the electrostatictransfer head, as described in more detail in FIGS. 15A-15E. This mayallow for some alignment tolerance of the electrostatic transfer headassembly.

Following the formation of discrete and laterally separate LED devices156, the sacrificial release layer 126 may be removed. FIG. 11A iscross-sectional side view illustrations of an array of LED devices 156supported by an array of stabilization posts 132 after removal of thesacrificial release layer in accordance with an embodiment of theinvention. In the embodiment illustrated, sacrificial release layer 126is completely removed resulting in an open space below each LED device156. A suitable etching chemistry such as HF vapor, or CF₄ or SF₆ plasmamay used to etch the SiO₂ or SiN_(x) sacrificial release layer 126. Inan embodiment, the array of LED devices 156 is on the array ofstabilization posts 132, and supported only by the array ofstabilization posts 132. In the embodiment illustrated, passivationlayer 120 is not removed during removal of the sacrificial release layer126. In an embodiment, passivation layer 120 is formed of Al₂O₃, and aSiO₂ or SiN_(x) sacrificial release layer 126 is selectively removedwith vapor HF.

Still referring to FIG. 11A, the LED device includes an active layer 108between a first current spreading layer pillar 118 and a second currentspreading layer 104, where the first current spreading layer pillar 118is doped with a first dopant type and the second current spreading layer104 is doped with a second dopant type opposite the first dopant type. Afirst cladding layer 110 is between the first current spreading layerpillar 118 and the active layer 108. A second cladding layer 106 isbetween the second current spreading layer 104 and the active layer 108.The first current spreading layer pillar protrudes away from the firstcladding layer 110 and the first cladding layer 110 is wider than thefirst current spreading layer pillar 118. In an embodiment, the firstcurrent spreading layer pillar 118 is a bottom current spreading layerpillar, the first cladding layer 110 is a bottom cladding layer, thesecond cladding layer 106 is a top cladding layer, and the secondcurrent spreading layer is a top current spreading layer of the LEDdevice. As shown, the passivation layer 120 may span along a bottomsurface of the bottom cladding layer 110 and sidewalls of the bottomcurrent spreading layer pillar 118. An opening is formed in thepassivation layer 120 on a bottom surface of the bottom currentspreading layer pillar 118. The bottom conductive contact 124 is formedwithin the opening in the passivation layer and in electrical contactwith the bottom current spreading layer pillar 118. In an embodiment,the bottom conductive contact is not in direct electrical contact withthe bottom cladding layer 110. In an embodiment, a top surface 162 ofthe top current spreading layer 104 is wider than a bottom surface ofthe bottom current spreading layer pillar 118. This may allow for alarger surface area for electrostatic pick up in addition to a structurefor confining current. In an embodiment, the LED device 156 is supportedby a post 132, and a surface area of a top surface of the post 132 isless than the surface area of the bottom current spreading layer pillar118.

In accordance with embodiments of the invention the LED devices 156 maybe micro LED devices. In an embodiment, an LED device 156 has a maximumwidth or length at the top surface 162 of top current spreading layer104 of 300 μm or less, or more specifically approximately 100 μm orless. The active area within the LED device 156 may be smaller than thetop surface 162 due to location of the bottom current spreading layerpillars 118. In an embodiment, the top surface 162 has a maximumdimension of 1 to 100 μm, or more specifically 3 to 20 μm. In anembodiment, a pitch of the array of LED devices 156 on the carriersubstrate may be (1 to 300 μm) by (1to 300 μm), or more specifically (1to 100 μm) by (1 to 100 μm), for example, 20 μm by 20 μm, 10 μm by 10μm, or 5 μm by 5 μm. In an exemplary embodiment, a pitch of the array ofLED devices 156 on the carrier substrate is 11 μm by 11 μm. In such anexemplary embodiment, the width/length of the top surface 162 isapproximately 9-10μm, and spacing between adjacent LED devices 156 isapproximately 1-2 μm. Sizing of the bottom current spreading layerpillars 118 may be dependent upon the width of the LED devices 156 andthe desired efficiency of the LED devices 156.

In the above exemplary embodiments, manners for forming LED devices 156including current spreading layer pillars are described. In the aboveembodiments, the current spreading layer pillars are formed from currentspreading layer 114. In other embodiments, the current spreading layerpillars may be formed from current spreading layer 104. Accordingly, insome embodiments the LED device pillar structure may be inverted. Thoughan inverted LED device pillar structure may not provide a larger contactarea for a transfer operation, such as described with regard to FIGS.15A-15E.

Referring now to FIGS. 11B-11D, top-bottom combination schematic viewillustrations are provided of LED devices with different sidewallconfigurations in accordance with embodiments of the invention. Asillustrated, each LED device may include mesa structure sidewalls 168and a current spreading layer pillar 118. Sidewalls may include avariety of configurations such as rectangular or square as shown in FIG.11B, triangular as shown in FIG. 11C, or circular as shown in FIG. 11D,amongst other shapes. Current spreading layer pillars 118 may alsoassume a variety of shapes including rectangular, square, triangular,circular, etc. In this manner, embodiments of the invention can be usedwith LED devices of various shapes, which may affect light extractionand EQE of the LED devices. As described above, the current spreadinglayer pillar 118 may protrude from a bottom of the LED device, or thedevice may be inverted and the current spreading layer pillar 118protrudes from a top of the LED device.

FIG. 12 is plot of radiative recombination as a function of distancefrom center of LED devices with different widths in accordance with anembodiment of the invention. Specifically, FIG. 12 illustratessimulation data for a 10 μm wide LED device and a 100 μm wide LEDdevice, as shown in solid lines, at operating current densities of 300nA/μm². The simulation data provided in FIG. 12 is based upon LEDdevices of constant width, without a pillar formation in the bottomcurrent spreading layer. Referring now specifically to the simulationdata for a 100 μm wide LED device, radiative recombination (resulting inlight emission) is at a peak value in the center of the LED deviceindicated by a distance of 0 μm. The peak value is relatively constantmoving away from the center until approximately 40 μm from center, wherea non-radiative zone begins and the radiative recombination begins totail off. Thus, this suggests that non-radiative recombination may occuralong exterior surfaces of the active layer (e.g. along sidewalls of theLED devices). The simulation data for the 100 μm wide LED devicesuggests that this non-radiative zone begins to occur at approximately10 μm from the exterior sidewalls, which may account for 20% of the LEDdevice being affected by the non-radiative recombination zone. Thesimulation data for the 10 μm wide LED device shows that the peak valueof radiative recombination (resulting in light emission) is at a peakvalue in the center of the LED device and immediately begins to degrademoving away from the center. Furthermore, the peak value of radiativerecombination is well below the peak value of the radiativerecombination for the 100 μm wide LED device, despite being driven atthe same operating current density of 300 nA/μm². This suggests thatnon-radiative recombination due to edge effects is dominant within the10 μm LED device, even within the center of the LED device. Thus, 100%of the LED device may be affected by the non-radiative recombinationzone resulting in lower efficiency or EQE.

It is believed that such non-radiative recombination may be the resultof defects, for example, that may be the result of forming mesa trenchesthrough the p-n diode layer to form an array of LED devices or a resultof surface states from dangling bonds at the terminated surface that canenable current flow and non-radiative recombination. Such non-radiativerecombination may have a significant effect on LED device efficiency,particularly at low current densities in the pre-droop region of the IQEcurve where the LED device is driven at currents that are unable tosaturate the defects. It is expected that as the LED device width (andactive layer width) is increased above 10μm the radiative recombination(resulting in light emission) in the center of the device increases asthe width increases until the peak value matches that in the 100 μm LEDdevice simulation data. In accordance with embodiments of the invention,the current injection area can be confined internally within the activelayer by forming the bottom current spreading layer in a pillarconfiguration, so that the current does not spread laterally to theexterior surfaces of the active layer where a larger amount of defectsmay be present. As a result, the amount of non-radiative recombinationdue to edge effects in the non-radiative zone near the exterior sidewallsurfaces of the active layer can be reduced or eliminated and efficiencyof the LED device increased.

FIG. 13 is a plot of internal quantum efficiency as a function ofcurrent density for exemplary 10 μm wide LED devices (quantum wellwidth) with current spreading layer pillars (p-doped) of differentwidths (1 μm, 2 μm, 4 μm, 6 μm, 8 μm, and 10 μm) in accordance withembodiments of the invention. As illustrated, IQE for the devicesincreases as the pillar size is reduced from 10 μm (no pillar) to 1 μm.This suggests that the pillar configuration is successful in confiningthe injection current internally within the LED devices, over very shortdistances (less than 10 μm), away from the sidewalls, particularly atlow current densities in the pre-droop region of the IQE curve where IQEcan be dominated by defects.

FIG. 14 is a plot of internal quantum efficiency as a function ofcurrent density for exemplary LED devices with current spreading layerpillars of different doping in accordance with embodiments of theinvention. Specifically, the simulation data provided in FIG. 14 is for10 μm wide LED devices (quantum well width) with 2 μm wide currentspreading layer pillars, where n-pillar simulation data is presentedalong with the 2 μm wide p-pillar data from FIG. 13. The simulation datasuggests IQE increases for both p-pillar and n-pillar configurations andthat the p-pillar configuration obtains a larger IQE.

FIGS. 15A-15E are cross-sectional side view illustrations of an array ofelectrostatic transfer heads 204 transferring LED devices 156, which maybe micro LED devices, from carrier substrate 140 to a receivingsubstrate 300 in accordance with an embodiment of the invention. FIG.15A is a cross-sectional side view illustration of an array of microdevice transfer heads 204 supported by substrate 200 and positioned overan array of LED devices 156 stabilized on stabilization posts 132 ofstabilization layer 130 on carrier substrate 140. The array of LEDdevices 156 is then contacted with the array of transfer heads 204 asillustrated in FIG. 15B. As illustrated, the pitch of the array oftransfer heads 204 is an integer multiple of the pitch of the array ofLED devices 156. A voltage is applied to the array of transfer heads204. The voltage may be applied from the working circuitry within atransfer head assembly 206 in electrical connection with the array oftransfer heads through vias 207. The array of LED devices 156 is thenpicked up with the array of transfer heads 204 as illustrated in FIG.15C. The array of LED devices 156 is then placed in contact with contactpads 302 (e.g. gold, indium, tin, etc.) on a receiving substrate 300, asillustrated in FIG. 15D. The array of LED devices 156 is then releasedonto contact pads 302 on receiving substrate 300 as illustrated in FIG.15E. For example, the receiving substrate may be, but is not limited to,a display substrate, a lighting substrate, a substrate with functionaldevices such as transistors or ICs, or a substrate with metalredistribution lines.

In accordance with embodiments of the invention, heat may be applied tothe carrier substrate, transfer head assembly, or receiving substrateduring the pickup, transfer, and bonding operations. For example, heatcan be applied through the transfer head assembly during the pick up andtransfer operations, in which the heat may or may not liquefy LED devicebonding layers. The transfer head assembly may additionally apply heatduring the bonding operation on the receiving substrate that may or maynot liquefy one of the bonding layers on the LED device or receivingsubstrate to cause diffusion between the bonding layers.

The operation of applying the voltage to create a grip pressure on thearray of LED devices can be performed in various orders. For example,the voltage can be applied prior to contacting the array of LED deviceswith the array of transfer heads, while contacting the LED devices withthe array of transfer heads, or after contacting the LED devices withthe array of transfer heads. The voltage may also be applied prior to,while, or after applying heat to the bonding layers.

Where the transfer heads 204 include bipolar electrodes, an alternatingvoltage may be applied across a pair of electrodes in each transfer head204 so that at a particular point in time when a negative voltage isapplied to one electrode, a positive voltage is applied to the otherelectrode in the pair, and vice versa to create the pickup pressure.Releasing the array of LED devices from the transfer heads 204 may beaccomplished with a varied of methods including turning off the voltagesources, lowering the voltage across the pair of electrodes, changing awaveform of the AC voltage, and grounding the voltage sources.

Referring now to FIGS. 16A-16B, in an embodiment, an array of LEDdevices is transferred and bonded to a display substrate. For example,the display substrate 302 may be a thin film transistor (TFT) displaysubstrate (i.e. backplane) similar to those used in active matrix OLEDdisplay panels. FIG. 16A is a top view illustration of a display panel1600 in accordance with an embodiment of the invention. FIG. 16B is aside-view illustration of the display panel 1600 of FIG. 16A taken alonglines X-X and Y-Y in accordance with an embodiment of the invention. Insuch an embodiment, the underlying TFT substrate 300 may include workingcircuitry (e.g. transistors, capacitors, etc.) to independently driveeach subpixel 328. Substrate 300 may include a non-pixel area and apixel area 304 (e.g. display area) including subpixels 328 arranged intopixels. The non-pixel area may include a data driver circuit 310connected to a data line of each subpixel to enable data signals (Vdata)to be transmitted to the subpixels, a scan driver circuit 312 connectedto scan lines of the subpixels to enable scan signals (Vscan) to betransmitted to the subpixels, a power supply line 314 to transmit apower signal (Vdd) to the TFTs, and a ground ring 316 to transmit aground signal (Vss) to the array of subpixels. As shown, the data drivercircuit, scan driver circuit, power supply line, and ground ring are allconnected to a flexible circuit board (FCB) 313 which includes a powersource for supplying power to the power supply line 314 and a powersource ground line electrically connected to the ground ring 316. It isto be appreciated, that this is one exemplary embodiment for a displaypanel, and alternative configurations are possible. For example, any ofthe driver circuits can be located off the display substrate 300, oralternatively on a back surface of the display substrate 300. Likewise,the working circuitry (e.g. transistors, capacitors, etc.) formed withinthe substrate 300 can be replaced with micro chips 350 bonded to the topsurface of the substrate 300 as illustrated in FIG. 16C.

In the particular embodiment illustrated, the TFT substrate 300 includesa switching transistor T1 connected to a data line from the drivercircuit 310 and a driving transistor T2 connected to a power lineconnected to the power supply line 314. The gate of the switchingtransistor T1 may also be connected to a scan line from the scan drivercircuit 312. A patterned bank layer 326 including bank openings 327 isformed over the substrate 300. In an embodiment, bank openings 327correspond to subpixels 328. Bank layer 326 may be formed by a varietyof techniques such as ink jet printing, screen printing, lamination,spin coating, CVD, PVD and may be formed of opaque, transparent, orsemitransparent materials. In an embodiment, bank layer 326 is formed ofan insulating material. In an embodiment, bank layer is formed of ablack matrix material to absorb emitted or ambient light. Thickness ofthe bank layer 326 and width of the bank openings 327 may depend uponthe height of the LED devices 156 transferred to and bonded within theopenings, height of the electrostatic transfer heads, and resolution ofthe display panel. In an embodiment, exemplary thickness of the banklayer 326 is between 1 μm-50 μm.

Electrically conductive bottom electrodes 342, ground tie lines 344 andground ring 316 may optionally be formed over the display substrate 300.In the embodiments illustrated an arrangement of ground tie lines 344run between bank openings 327 in the pixel area 304 of the display panel1600. Ground tie lines 344 may be formed on the bank layer 326 oralternative, openings 332 may be formed in the bank layer 326 to exposeground tie lines 344 beneath bank layer 326. In an embodiment, groundtie liens 344 are formed between the bank openings 327 in the pixel areaand are electrically connected to the ground ring 316 or a ground linein the non-display area. In this manner, the Vss signal may be moreuniformly applied to the matrix of subpixels resulting in more uniformbrightness across the display panel 1600.

A passivation layer 348 formed around the LED devices 156 within thebank openings 327 may perform functions such as preventing electricalshorting between the top and bottom electrode layers 318, 342 andproviding for adequate step coverage of top electrode layer 318 betweenthe top conductive contacts 152 and ground tie lines 344. Thepassivation layer 348 may also cover any portions of the bottomelectrode layer 342 to prevent possible shorting with the top electrodelayer 318. In accordance with embodiments of the invention, thepassivation layer 348 may be formed of a variety of materials such as,but not limited to epoxy, acrylic (polyacrylate) such as poly(methylmethacrylate) (PMMA), benzocyclobutene (BCB), polymide, and polyester.In an embodiment, passivation layer 348 is formed by ink jet printing orscreen printing around the LED devices 156 to fill the subpixel areasdefined by bank openings 327.

Top electrode layer 318 may be opaque, reflective, transparent, orsemi-transparent depending upon the particular application. In topemission display panels the top electrode layer 318 may be a transparentconductive material such as amorphous silicon, transparent conductivepolymer, or transparent conductive oxide. Following the formation of topelectrode layer 318 an encapsulation layer 346 is formed over substrate300. For example, encapsulation layer 346 may be a flexibleencapsulation layer or rigid layer. In accordance with some embodimentsof the invention, a circular polarizer may not be required to suppressambient light reflection. As a result, display panels 3700 in accordancewith embodiments of the invention may be packaged without a circularpolarizer, resulting in increased luminance of the display panel.

In an embodiment, one or more LED devices 156 are arranged in a subpixelcircuit. A first terminal (e.g. bottom conductive contact) of the LEDdevice 156 is coupled with a driving transistor. For example, the LEDdevice 156 can be bonded to a bonding pad coupled with the drivingtransistor. In an embodiment, a redundant pair of LED devices 156 arebonded to the bottom electrode 342 that is coupled with the drivingtransistor T2. The one or more LED devices 156 may be any of the LEDdevices described herein including a confined current injection area. Aground line is electrically coupled with a second terminal (e.g. topconductive contact) for the one or more LED devices.

A current can be driven through the one or more LED devices, forexample, from the driving transistor T2. In a high side driveconfiguration the one or more LED devices may be on the drain side of aPMOS driver transistor or a source side of an NMOS driver transistor sothat the subpixel circuit pushes current through the p-terminal of theLED device. Alternatively, the subpixel circuit can be arranged in a lowside drive configuration in which case the ground line becomes the powerline and current is pulled through the n-terminal of the LED device.

In accordance with embodiments of the invention, the subpixel circuitmay operate at comparatively low currents or current densities in thepre-droop range of the characteristic efficiency curve of the LEDdevices, or near a maximum efficiency value past the pre-droop range.Thus, rather than increasing the size of the LED devices to increaseefficiency, the effective size of the current injection area is confinedin order to increase the current density within the LED device. Inembodiments where the LED devices are utilized in display applications,as opposed to high-powered applications, the LED devices can operate atcomparatively lower current ranges, where a slight increase in currentdensity may result in a significant improvement in IQE and EQE of theLED devices.

In an embodiment, a subpixel circuit comprises a driving transistor, afirst terminal (e.g. bottom electrically conductive contact) of an LEDdevice with confined current injection area is coupled with the drivingtransistor, and a ground line is coupled with a second terminal (e.g.top electrically conductive contact) of the LED device. In anembodiment, the LED device is operated by driving a current through theLED device in response to sending a control signal to the drivingtransistor. In some embodiments, the current may range from 1 nA-400 nA.In an embodiment, the current ranges from 1 nA-30 nA. In an embodiment,an LED device is operated with a current from 1 nA-30 nA in a displayhaving a 400 pixel per inch (PPI) resolution. In an embodiment, thecurrent ranges from 200 nA-400 nA. In an embodiment, an LED device isoperated with a current from 200 nA-400 nA in a display having a 100 PPIresolution. In some embodiments, an LED device is operated with aconfined current density from 0.001 A/cm² to 40 A/cm². In an embodiment,the current density ranges from 0.001 A/cm² to 3 A/cm². In anembodiment, such a current density range may be applicable to a displayhaving a 400 PPI resolution. In an embodiment, the current densityranges from 0.2 A/cm² to 4 A/cm². In an embodiment, such a currentdensity range may be applicable to a display having a 100 PPIresolution.

The following examples are provided to illustrate the effect of currentconfinement, and the relationship of efficiency, current and currentdensity for LED devices in accordance with embodiments of the invention.In accordance with embodiments of the invention, a designer may select adesired efficiency and luminance of an LED device with a characteristicefficiency curve, such as the exemplary efficiency curve illustrated inFIG. 1. Upon selecting the desired efficiency and luminance, thedesigner may tune the operating current and size of the confined currentinjection area (e.g. approximate current spreading layer pillar width)within the LED device to achieve the desired efficiency.

EXAMPLE 1

In one embodiment, a display panel is a 5.5 inch full high definitiondisplay with 1920×1800 resolution, and 400 pixels per inch (PPI)including a 63.5 μm RGB pixel size. To achieve a 300 Nit output (white)with LED devices having a 10% EQE, the display panel uses approximately10 nA-30 nA of current per LED, assuming one LED per subpixel. For anLED device with a 10 μm×10 μm confined current injection area thiscorresponds to a current density of 0.01 A/cm²-0.03 A/cm². This is wellbelow the normal or designed operating conditions for standard LEDs.

EXAMPLE 2

In an embodiment, the parameters of Example 1 are the same, with asmaller 1 μm×1 μm confined current injection area. With this reducedcurrent injection area the corresponding current density increases to 1A/cm²-3 A/cm². Thus, Example 2 illustrates that at operating currents of10 nA-30 nA, small changes in current injection area from 10 μm×10 μm to1 μm×1 μm can have a significant effect on current density. In turn, thechange in current density may affect efficiency of the LED device.

EXAMPLE 3

In one embodiment, a display panel is a 5.5 inch full high definitiondisplay with 1920×1800 resolution, and 400 pixels per inch (PPI)including a 63.5 μm RGB pixel size. Each subpixel includes an LED devicewith a 10 μm×10 μm confined current injection area. Luminance ismaintained at 300 Nit output (white). In this example, it is desired toachieve a 40% EQE. With this increased efficiency, lower operatingcurrents may be used. In an embodiment, an operating current of 3 nA-6nA per LED is selected. With these parameters an LED device with a 10μm×10 μm confined current injection area operates at 0.003 A/cm²-0.006A/cm², and an LED device with a 1 μm×1 μm confined current injectionarea operates at 0.3 A/cm²-0.6 A/cm².

EXAMPLE 4

In one embodiment, a display panel is a 5.5 inch display with a lowerresolution of 100 PPI including a 254 μm RGB pixel size. To achieve a300 Nit output (white) with LED devices having a 10% EQE, the displaypanel uses a higher operating current of approximately 200 nA-400 nA ofcurrent per LED, assuming one LED per subpixel. For an LED device with a10 μm×10 μm confined current injection area this corresponds to acurrent density of 0.2 A/cm²-0.4 A/cm². A 1 μm×1 μm confined currentinjection area corresponds to a current density of 20 A/cm²-40 A/cm²,and a 3 μm×3 μm confined current injection area corresponds to a currentdensity of 2 A/cm²-4 A/cm². Thus, Example 4 illustrates that with lowerresolution displays, there is a smaller density of LED devices, andhigher operating currents are used to achieve a similar brightness (300Nit) as higher resolution displays.

EXAMPLE 5

In one embodiment, a display panel has 716 PPI including a 35 μm RGBpixel size. To achieve a 300 Nit output (white) with LED devices havinga 10% EQE, the display panel uses an operating current of approximately4-7 nA. With these parameters an LED device with a 10 μm×10 μm confinedcurrent injection area operates at 0.004 A/cm²-0.007 A/cm², and an LEDdevice with a 1 μm×1 μm confined current injection area operates at 0.4A/cm²-0.7 A/cm².

EXAMPLE 6

In another embodiment the required brightness of the display isincreased to 3000 Nit. In all examples above the required current wouldincrease about 10× if the same EQE is targeted. Subsequently, thecurrent density would also increase 10× for the above examples. In oneembodiment the required operating brightness is a range from 300 Nit to3000 Nit. The current and subsequently the current density would span arange of 1-10× the 300 Nit range. In the case of Examples 1 and 2(above) where now 300 Nit to 3000 Nit is required, an LED device with a10 μm×10 μm confined current injection area operates at a currentdensity of 0.01 A/cm²-0.3 A/cm² and an LED device with a 1 μm×1 μmconfined current injection area operates at 1 A/cm²-30 A/cm².

In each of the above exemplary embodiments, the brightness of thedisplay is such that the LED devices are operating at very low currentdensities that are not typical of standard LEDs. The typical performanceof standard LEDs show low IQEs at current densities below 1 A/cm². Inaccordance with embodiments of the invention, the current injection areais confined such that the current density can be increased to allowoperation of the LED devices in a current density regime where IQE, andEQE, are optimized.

In an embodiment, the LED devices are bonded to a display substrate in adisplay area of the display substrate. For example, the displaysubstrate may have a pixel configuration, in which the LED devicesdescribed above are incorporated into one or more subpixel arrays. Thesize of the LED devices may also be scalable with the available area ofthe subpixels. In some embodiments, the LED devices are bonded to adisplay substrate having a resolution of 100 PPI or more. In theExamples provided above, exemplary red-green-blue (RGB) pixel sizes of35 μm were described for a display having 716 PPI, RGB pixels sizes of63.5 μm were described for a display having 400 PPI, and RGB pixelssizes of 254 μm were described for a display having 100 PPI. In someembodiments, the LED devices have a maximum width of 100 μm or less. Asdisplay resolution increases, the available space for LED devicesdecreases. In some embodiments, the LED devices have a maximum width of20 μm or less, 10 μm or less, or even 5 μm or less. Referring back tothe above discussion with regard to FIG. 12, a non-radiative zone mayoccur along exterior surfaces of the active layer (e.g. along sidewallsof the LED devices), affecting efficiency of the LED devices. Inaccordance with embodiments of the invention, a current spreading layeris formed in a pillar configuration, in which the current spreadinglayer pillar protrudes from a cladding layer, and the width of thecurrent spreading layer pillar may be adjusted relative to the width ofthe LED device (e.g. width of the active layer) in order to confinecurrent within an interior of the active layer. In some embodiments,current spreading layer pillars have a width between 1 and 10 μm. In anembodiment, current spreading layer pillars have a width or diameter ofapproximately 2.5 μm.

FIG. 17 illustrates a display system 1700 in accordance with anembodiment. The display system houses a processor 1710, data receiver1720, a display 1730, and one or more display driver ICs 1740, which maybe scan driver ICs and data driver ICs. The data receiver 1720 may beconfigured to receive data wirelessly or wired. Wireless may beimplemented in any of a number of wireless standards or protocolsincluding, but not limited to, Wi-Fi (IEEE 802.11 family), WiMAX (IEEE802.16 family), IEEE 802.20, long term evolution (LTE), Ev-DO, HSPA+,HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA, TDMA, DECT, Bluetooth,derivatives thereof, as well as any other wireless protocols that aredesignated as 3G, 4G, 5G, and beyond. The one or more display driver ICs1740 may be physically and electrically coupled to the display 1730.

In some embodiments, the display 1730 includes one or more LED devices156 that are formed in accordance with embodiments of the inventiondescribed above. Depending on its applications, the display system 1700may include other components. These other components include, but arenot limited to, memory, a touch-screen controller, and a battery. Invarious implementations, the display system 1700 may be a television,tablet, phone, laptop, computer monitor, kiosk, digital camera, handheldgame console, media display, ebook display, or large area signagedisplay.

FIG. 18 illustrates a lighting system 1800 in accordance with anembodiment. The lighting system houses a power supply 1810, which mayinclude a receiving interface 1820 for receiving power, and a powercontrol unit 1830 for controlling power to be supplied to the lightsource 1840. Power may be supplied from outside the lighting system 1800or from a battery optionally included in the lighting system 1800. Insome embodiments, the light source 1840 includes one or more LED devices156 that are formed in accordance with embodiments of the inventiondescribed above. In various implementations, the lighting system 1800may be interior or exterior lighting applications, such as billboardlighting, building lighting, street lighting, light bulbs, and lamps.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming an LED device includingany one of a confined current injection area. Although the presentinvention has been described in language specific to structural featuresand/or methodological acts, it is to be understood that the inventiondefined in the appended claims is not necessarily limited to thespecific features or acts described. The specific features and actsdisclosed are instead to be understood as particularly gracefulimplementations of the claimed invention useful for illustrating thepresent invention.

What is claimed is:
 1. An LED device comprising: a top current spreadinglayer; a top cladding layer below the top current spreading layer; anactive layer below the top cladding layer; a bottom cladding layer belowthe active layer, the bottom cladding layer including a bottom surface;sidewalls spanning the top current spreading layer, the top claddinglayer, the active layer, and the bottom cladding layer, wherein thebottom surface of the bottom cladding layer extends between thesidewalls; and a singular bottom current spreading layer pillar belowthe bottom cladding layer and in direct contact with the bottom claddinglayer, wherein the bottom current spreading layer pillar is centrallylocated at and protrudes from the bottom cladding layer, and the bottomsurface of the bottom cladding layer that extends between the sidewallsis wider than a maximum width of the bottom current spreading layerpillar, and the bottom current spreading layer pillar is doped with afirst dopant type and the second current spreading layer is doped with asecond dopant type opposite the first dopant type.
 2. The LED device ofclaim 1, further comprising a passivation layer spanning along thebottom surface of the bottom cladding layer and sidewalls of the bottomcurrent spreading layer pillar.
 3. The LED device of claim 2, furthercomprising an opening in the passivation layer on a bottom surface ofthe bottom current spreading layer pillar opposite the bottom claddinglayer.
 4. The LED device of claim 3, further comprising a conductivecontact within the opening in the passivation layer and in electricalcontact with the bottom current spreading layer pillar.
 5. The LEDdevice of claim 4, wherein the conductive contact is not in directelectrical contact with the bottom cladding layer.
 6. The LED device ofclaim 1, wherein the top current spreading layer is wider than thebottom current spreading layer pillar.
 7. The LED device of claim 1,wherein the bottom current spreading layer pillar is doped with ap-dopant.
 8. The LED device of claim 7, wherein the bottom currentspreading layer pillar comprises GaP and the bottom cladding layercomprises a material selected from the group consisting of AlInP,AlGaInP, and AlGaAs.
 9. The LED device of claim 1, wherein the activelayer of the LED device has a maximum width of 100 μm or less, and thebottom current spreading layer pillar maximum width is 10 μm or less.10. The LED device of claim 1, wherein the active layer of the LEDdevice has a maximum width of 20 μm or less, and the bottom currentspreading layer pillar maximum width is 10 μm or less.
 11. The LEDdevice of claim 1, wherein the active layer includes 1-3 quantum welllayers.
 12. A display system comprising: a display substrate including adisplay area; an LED device bonded to the display substrate within thedisplay area and in electrical connection with working circuitry in thedisplay substrate, the LED device comprising: a top current spreadinglayer; a top cladding layer below the top current spreading layer; anactive layer below the top cladding layer; a bottom cladding layer belowthe active layer, the bottom cladding layer including a bottom surface;sidewalls spanning the top current spreading layer, the top claddinglayer, the active layer, and the bottom cladding layer, wherein thebottom surface of the bottom cladding layer extends between thesidewalls; and a singular bottom current spreading layer pillar belowthe bottom cladding layer and in direct contact with the bottom claddinglayer, wherein the bottom current spreading layer pillar is centrallylocated at and protrudes from the bottom cladding layer, and the bottomsurface of the bottom cladding layer that extends between the sidewallsis wider than a maximum width of the bottom current spreading layerpillar, and the bottom current spreading layer pillar is doped with afirst dopant type and the second current spreading layer is doped with asecond dopant type opposite the first dopant type.
 13. The displaysystem of claim 12, wherein the LED device is in electrical connectionwith a micro chip bonded to the display substrate within the displayarea.